1. Field of Invention
The present invention generally relates to object detection, recognition and location systems where sensor performance is severely limited due to size, weight and power constraints.
2. Background of the Invention
Current aerial surveillance and targeting systems are subject to the same fundamental design constraints: 1) Use of IR detection sensors for night-time and all weather capability; 2) some form of gimbaling mechanism to reorient the position of the sensor either to seek out new objects or to maintain the line of sight of the sensor relative to the object of interest while compensating for the forward motion of the airborne platform or movement of the object itself; 3) stabilization to isolate the sensor from vibration, pitch, yaw and roll motions as well as air turbulence.
Uncooled IR sensors possess approximately ten percent of the sensitivity to light of a normal day light television camera. Sensitivity limitations ultimately affect the quality and the detail of the images which may be obtained. Current approaches compensate for this limitation by using multiple sensors arranged in arrays and by cooling the head of the IR detector to make it more sensitive to IR radiation. These approaches trade increased sensitivity at the cost of increased size and weight.
The prime example of this is the Forward Looking Infra-Red or FLIR. A FLIR design usually contains other features: a laser range finder, a day time light television camera, optics to integrate, amplify and clarify images from the optical detectors, a means to track and gimbol the FLIR to objects of interest and a means to stabilize the unit from the platform's engine vibrations, changes in attitude due to aerial maneuvering and buffeting due to air turbulence. FLIRS are neither particularly miniature or light weight. One of the smallest commercially available FLIRs, the Microstar® manufactured by the FLIR Systems Company is 34.3 centimeters high and weighs 11.6 kilograms.
Existing airborne detection and targeting systems whether manned or unmanned also require the intervention of a trained operator to direct a gimbaled sensor to the area of interest, detect and identify the objects of interest, determine their location, and communicate the information to a ground based command post for processing and decision making. This human data processing takes place in real time. Support for this mission usually requires a helicopter or fixed wing aircraft and two people—a pilot and sensor system operator or an unmanned air vehicle (UAV).
UAVs though unmanned, fall into the size range of manned aircraft. For example the General Atomics Predator® has a length of 8.7 meters and a wingspan exceeding 15.5 meters. The smaller, General Atomics Prowler has a length of 4.9 meters and a wingspan of 7.74 meters. These planes are capable of carrying payloads in the range of 130 to 220 kilogram and remaining aloft for more than 24 hours.
These systems, whether manned or unmanned, require highly trained personnel, are expensive to operate, and at times put the human operators in harms way. In addition, they require complex logistics systems to service, maintain and support the sensor, the airframe which carries it, and the air crew. In the case of UAVs, this includes a ground control station capable of controlling the UAV, directing the sensor, and receiving transmitted images in real time.
By contrast a MUAV is lighter, 4.5 kilograms or less, has much lower capital, and operating costs than a conventional aircraft or UAV. It does not require a launch pad or airport, and can be carried in a car for instant deployment when needed. Like UAVs, it does not put its operators in harms way for dangerous missions. MUAV surveillance systems are however, severely constrained by their own weight, size, and power limitations. By way of context, the Microstar® FLIR mentioned above is more than twice the weight of a MUAV.
In turn, MUAVs tactical capabilities are limited due to the performance of existing state of the art technology for small, light-weight night vision, infrared (IR) sensors. To meet weight requirements, Single not arrays of IR sensors must be employed. The sensor must be uncooled because use of a cryostatic device to cool the sensor would impose unacceptable power and weight requirements. Today's state of the art uncooled sensors have limited resolution, requiring a narrow field of view (FOV) in the range of 15 degrees or a telephoto lens to resolve ground objects. In addition, weight and power limitations of an MUAV preclude the use of a three axis, stabilized, gimbaled platform to direct the sensor.
The use of a fixed, narrow FOV sensor imposes several limitations on the system which inhibit its ability to effectively perform its mission. The operator must be able to recognize the objects in the FOV of the sensor to discern targets of interest from non targets and differentiate “friend” from “foe”. At the 35 mile per hour baseline speed of current MUAV's, the operator will experience a “Soda Straw” effect similar to that experienced in looking out the side of a fast moving vehicle with a pair of powerful binoculars. Objects do not remain in the field of view long enough for the operator to recognize objects. To illustrate this limitation, an MUAV equipped with a representative state of the art uncooled IR sensor with a field of view (FOV) of 15 degrees, flying at 60 kilometers per hour (17 meters per second) at an altitude of 100 meters would have an effective visual area of less than 50 meters in diameter. The time required for a human operator to recognise an object within the FOV is 5 to 9 seconds. Given these parameters, the maximum time an object would be in the FOV is 3 seconds, less if it is not located along the diameter of the FOV. In either case there is insufficient time in the FOV for operator recognition.
Often, the mission is to search an area for potential objects where no prior information is known about approximate location of objects of interest. The vehicle is then forced to fly a search pattern. The time required to search even a small area is excessive if the MUAV is forced to fly at speeds slow enough to enable the operator to recognize targets because of the narrow sensor footprint over the ground. Using the speed, altitude and FOV values of the previous example, and assuming that the time to execute a 180 degree turn is 30 seconds, the search time for a 10 square kilometer area would be almost 5 hours at speeds where imagery is barely recognizable or non-recognizable to the operator.
Once an object of interest is identified, it is desirable to loiter over its location and observe. A circular loiter pattern is not feasible with a fixed camera because the bank angle of the aircraft would be greater than 15 degrees. With a FOV of 15 degrees and an altitude of 160 meters, the resulting visual footprint of the state of the art uncooled IR sensor is approximately 80 meters. The object can be observed with a racetrack or figure eight pattern, but these patterns allow the object to be within the FOV for only approximately 5% of the time in a typical holding pattern.
Use of an on-board laser range finder is not possible because of payload weight constraints of MAUVs. Current systems employed on MAUVs therefore determine the target location using the Global Positioning System coordinates of the air vehicle and an algorithm which triangulates the location of the object of interest using altitude and the LOS angle from vertical These algorithms assume that the terrain is flat. Significant errors can be introduced in the situation where the terrain is in fact mountainous or hilly.